Various tests have been used to determine the operating condition of the thermostat in the cooling system of internal combustion engines. Conceptually, the simplest of these tests involves removal of the thermostat from the cooling system, placing it in a heated pan of liquid with a thermometer, and visually observing mechanical motion indicative of opending of the thermostat and the temperature at which it occurred. Naturally, this test is unsatisfactory since it requires the breaking of a liquid seal and an off-vehicle test, and the potential for fouling operation on reinstallation; and the temperature may not vary sufficiently to use it as a visual indication of operation. For certain cooling systems, in dependence upon the overall parameters thereof, it has been known to visually monitor temperature and pressure gages affixed to the cooling system essentially at the thermostat housing, to determine the indicated temperature at which an apparent pressure drop seems to be signaling the opening of the thermostat. However, in some systems, the pressure drop is not sufficiently significant so as to be a viable test, particularly when done visually. And, the pressure may vary due to other factors sufficiently to mask the anticipated indication.
In modern vehicle diagnostics, it is desirable to be able to measure as many parameters as possible without removing parts from the engine, and with a minimum of attachments to the engine. When complex diagnostics are involved, it is desirable that the diagnostic equipment be operative to test the widest variety of vehicles and/or engines. Similarly, automated diagnosis is feasible in a practical sense only if the versatility and simplicity of performance of the test permit cost savings as well as providing reliable measurement information about the vehicles and/or engines under test.